https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97976
Peter Bisroev <peter at int19h dot net> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|INVALID |FIXED
--- Comment #5 from Peter Bisroev <peter at int19h dot net> ---
Thank you for your explanation Richard, it makes perfect sense to me now. GCC
is being insanely clever here!
However I still think there is something not 100% right. If I change
containsBackwards() function to look like this:
////////////////////
int containsBackwards(const uint8_t* p, uint8_t target)
{
for (;;)
{
if (*p == target)
{
return 1;
}
--p;
if (!p)
{
return 0;
}
}
}
////////////////////
I apologize in advance as I know the code is far from ideal and is definitely a
corner case, but unless I am mistaken there is no undefined behavior here
assuming p is always valid at the time of the call and it is just to illustrate
the example. So for the above gcc 10.1 generates the following:
////////////////////
containsBackwards(unsigned char const*, unsigned char):
mov eax, 1
ret
////////////////////
So my question is, is gcc allowed to assume that the target will always match?
Additionally, if I make this function a bit safer and unless I am mistaken
without any undefined behavior:
////////////////////
int containsBackwardsSafe(const uint8_t* p, uint8_t target)
{
if (!p)
{
return 0;
}
for (;;)
{
if (*p == target)
{
return 1;
}
--p;
if (!p)
{
return -1;
}
}
}
////////////////////
Then gcc 10.1 generates the following:
////////////////////
containsBackwardsSafe(unsigned char const*, unsigned char):
xor eax, eax
test rdi, rdi
setne al
ret
////////////////////
Which matches the generated output of the original function. However unless I
am mistaken the result here is supposed to be different to the original
function:
* If p == NULL, return 0.
* If target matched while p was valid, return 1.
* If target did not match while p was valid return -1.
You can see these above examples at Compiler Explorer here:
https://godbolt.org/z/ha1c1h
I apologize if there is something critical I am missing here so I hope I am not
wasting your time.
Thank you!
--peter
